I pulled this engine apart and in the process of re-assembly, trying to get it running by tonight. The round dot on the sprocket is either 1/2 tooth above or if adjusted again, 1/2 tooth below the mark on the casing. I'm the 2nd owner, and to my knowledge, the head has never been machined, chain is tight. What should I do here? 1/2 tooth makes a huge difference on the crank timing mark alignment.

 

Racing engines need accurate cam timing. You don't have a racing engine, and half a tooth is regarded as "near enough" by the makers, otherwise they would provide some way of getting it more accurate. I have written this on here before, but a known tuning tip on BMC a series engines in the 1960s was to fit the cam one tooth out, which gave better performance. British Leyland, who took over BMC, actually incorporated the one tooth alteration as standard, on A series engines they made for the MG Metro. When faced with the half a tooth out problem I fit the chain so it will get closer to correct as the chain wears and stretches.